Construction of stator elements of turbines, compressors, or like machines



Feb. 5, 1952 R c, MCLEOD 2,584,899

CONSTRUCTION OF STA'IOR ELEMENTS OF TURBINES, COMPRESSORS OR LIKE MACHINES 2 SHEETS-SHEET 1 Filed Jan. 14, 1946 Feb. 5, 1952 R c, LEQD 2,584,899 CONSTRUCTION OF STATOR ELEM TS OF TURBINES, COMPRESSORS OR LIKE MACHINES Filed Jan. 14, 1946 2 SYIEETSSHEET 2 my; Mm

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Patented Feb. 5, 1952 CONSTRUCTION OF STATOR ELEMENTS OF TURBINES, COMPRESSORS, OR LIKE MA- CHINES Roderick Cristal! McLeod, Leicester, England,

assignor to Power Jets (Research & Development) Limited, London, England Application January 14, 1946, Serial No. 641,148 In Great Britain January 23, 1945 12 Claims.

This invention relates to the construction of stator elements of turbines, compressors or like machines and has for its principal object to provide a construction of working fluid channel for the stator portions of such machines which will simplify the problem of maintaining at reasonably small value the nominal clearance between a wall of such a channel and an element which has to be contained with a clearance therein, notwithstanding that the parts are subject to expansion and contraction due to temperature variation.

The invention is deemed to be particularly applicable to, and has special value in machines in which a large temperature variation is experienced, and thus has particular application to internal combustion gas turbines operating at high temperatures, for example axial flow gas turbines such as are used in aircraft power plants, in which the problem of maintaining a small clearance between the working fluid channel and moving parts therein arises in its most acute form. The invention has been evolved primarily to meet this problem as it arises in turbines of that type in the case of the clearance between the tips of the turbine rotor blades and their stator shroud, and will be described in relation to that particular case without prejudice to the generality of the scope of the appended claims. In turbines of the type in question the stator construction presents a smaller mass than the rotor and thus heats more rapidly, with the result that the stator diameter and therefore the blade tip clearance increases on starting, and emciency falls until the rotor becomes heated. In the case of a gas turbine this difllculty cannot be met by a preliminary warming-up as with steam turbines; on the contrary, it is in the nature of the machine that a rapid temperature rise is inevitable. Conversely, when the turbine is shut down, the stator, being more favourably situated for cooling as well as having a smaller mass, cools more rapidly than the rotor so that unless adequate tip clearance is allowed in the first place, the stator will shrink on to the blades, causing serious damage. These difiiculties become the more acute as the rotor diameter is increased and the length or relative length of the blades is reduced, since in such a case the ratio of the blade tip clearance to blade height may reach a prohibitive value from the point of view of working efliciency.

A further object of the invention is to enable the stator bladlng, whether as a whole or as individual blades or pluralities of blades, to be re- 2 moved or replaced simply and easily. Yet another object is the provision of a stator construction especially well adapted to the requirement 0f multistage machines such that the stator as a whole can be built up in the course of assembly of a complete machine.

According to the invention there is provided a turbine, compressor, or like machine, especially an internal combustion turbine operating at high temperatures, having a stator working fluid channel containing an element which has to have a clearance therein, both the channel and the element being required to be subject to thermal radial expansion and contraction, wherein a boundary of said channel is defined by shrouding embodying expansion joints effective in the peripheral direction such as will allow thermal expansion and contraction of the material of the shroud without a corresponding variation in the radius thereof, said shrouding being supported in relation to the channel assembly by means which control the radius of the shrouding independently of the expansion and contraction characteristic of the latter, so that at any time said radius depends upon the condition of said supporting means and not upon the temperature of the shroud. According to a supplementary feature of the invention, the channel and the element enclosed thereby being required to be subject to difierential expansion and contraction, the shrouding is constituted by a high temperature inner wall carried by a low temperature support made as a rigid unit which is screened from the channel by and operates within a smaller temperature range than the shrouding, the mounting of the inner wall on its support being such that the radius of the former is determined by the condition of the latter.

More specifically stated in relation to the particular problem of the turbine rotor and its stator shroud, the stator assembly comprises rotor blade shrouding constituted by a high temperature inner wall exposed to the temperature of the working gas flow and embodying expansion joints effective in the peripheral direction so that thermal expansion of the material of the shroud may take place without variation in the diameter thereof, and an outer low temperature casing made as a rigid unit which is cooled to operate within a smaller temperature range than the shrouding, the latter being fixed to said rigid casing so that its diameter is determined thereby. It will be appreciated that with such an arrangement the high temperature inner wall will be subject only to the same variations in diameter as the low temperature outer casing, so that the diameter of said inner wall may be maintained constant to a degree determined by the limits of temperature variation permitted to the outer casing, and it becomes possible, by allowing a cold blade tip clearance calculated only on the estimated expansion of the rotor, to eliminate the danger of the shroud ring shrinking into contact with the blades when cooling. It may be, however, in some cases that such clearance may still exceed an acceptable value when the turbine is being started from cold, due to the fact that the rotor heats relatively slowly. To meet this difficulty the rotor, according to a further feature of the invention, is also cooled so that the tip clearance at starting may the more closely approach that existing under running conditions.

According to further features of the invention, which may be used alone or jointly, the low temperature outer casing of the stator assembly is built up of a plurality of ring units (which may be continuous peripherally or may be built up in segments) interattached axially; these ring units by their juxtaposition locate the roots of stator blades in internal grooves presented by the assembly of rings; the high temperature inner wall of the stator assembly is built up (in the case of a multistage turbine) of a plurality of ring units; and the high temperature inner wall and low temperature outer casing, or their component ring units, have mutually interengaging formations by which the inner wall is rigidly locked to the outer casing.

In order to achieve the desired difference in temperature relationship between the inner wall and outer casing the invention further provides for cooling the latter. For this purpose, in accordance with a feature of the invention, the said inner wall is mounted on the outer casing in radially spaced relationship thereto and provision is made for a circulation of collant to take place through the cavity so provided.

The means employed for cooling the turbine rotor preferably includes ports and guides coupled to a source of compressed air and adapted to supply such air to and conduct it along one or more surfaces of the rotor, the preferred arrangement being one in which only a thin layer of rapidly changed air passes over the surface to be cooled. These requirements are readily met, if, as in accordance with a further feature of the invention, the body of the rotor is enclosed, at least at one side thereof, in a chamber which is main: tained at a pressure higher than that of the working gas stream and has outlets thereto. Further and more detailed features of the invention will appear from the following description with reference to the accompanying drawings, which illustrate by way of example the application of one constructional form of the invention to a gas turbine aircraft power plane embodying a two-stage axial flow turbine, and in which- Figure 1 is an axial section of part of such a power plant to indicate the relationship of the turbine thereto;

Figure 2 is an enlarged axial section of the turbine incorporated in said plant; and

Figure 3 is a detail of the inner high temperature shroud construction.

Referring more particularly to Figure 1 of the drawings, the power plant in which the invention has been illustrated comprises a multistage axial flow, centrifugal discharge compressor I of which only the final stages are shown, said compressor supplying air under pressure to an annular aircasing or combustion chamber 2 defined by inner and outer walls 3, 4, symmetrically surrounding a shaft 5 by which the compressor I is driven from a turbine generally indicated as 8. The atmosphere of the chamber I enclosed by the inner wall 3 of the aircasing 2 is maintained underpressure by supplying compressed air through a cooler 8 and a pipe connection 9 from the aircasing 2. The latter contains, peripherally spaced around its annulus, combustion elements comprising fuel nozzles or burners III directed into flame tubes I I, which discharge the working gases at the inlet nozzle I2 of the turbine. In this connection it may be mentioned that the outlets from the flame tubes II are peripherally flattened to form seg-.

ments of an annulus, thus affording a virtually continuous annular outlet to the turbine.

The preferred application of the invention is to a turbine which, as illustrated, has a working fluid passage that expands in the downstream direction. Further, the turbine nozzle and stator blade assembly are preferably built up by an assembly of ring elements of appropriately varying diameter which are mutually interengaging and supporting and constitute together the high temperature inner wall and low temperature outer casing of the invention, and are assembled by being entered from the wider end of the passage as the assembly progresses, each successive element to be so entered retaining the one before it. Stated in greater detail, the turbine nozzle and stator blade assembly is constructed as follows (see more particularly Figure 2) The first element of the rigid outer casing, which also forms the outer wall of the turbine nozzle ring, is a first casing ring I3, being the casing ring of least diameter, which is attached to the engine structure by bolts or the like and comprises, extending radially inwardy and outwardly at its forward end, a flange I4 and at its rearward edge internally at rebated groove and also at its rearward edge an external radially extending flange I5 for the attachment of the second casing ring. The nozzle ring stator blading comprises individual blades I6 each having a platform I! at its root, of rectangular or rhomboidal form. On the reverse side of the platform H from the blade proper is a rib or key I8 which lies in a corresponding axially directed or skewed groove formed in the ring I3 and this rib at its rear end has a tooth I9 to engage the rebated groove of the ring. Each blade is entered from the rear until the front edge of its platform is adjacent to the first mentioned flange I4 of the ring I3 and its tooth I9 is engaged; its rib I8 is now in the complementary groove in the ring l3. The second casing ring 20 is now positioned against the first ring I3 by a flange 2I whose inner end abuts against the rear edges of the teeth I9 of the blades I5, locking them in their rebated groove. The flange 2| at its outer part is extended forwardly at 22 to form an angle by which the outer edge of the flange I5 of the ring I3 is located, and also to meet the outer part of the flange I4, to which it is bolted and with which and the the first ring I3 it defines an annular air chamber 23 to which cooling air is supplied from the cooler 8 by way of a connection 24 to an inlet 25 associated with the chamber 23. The latter has outlets to the spaces between the ribs I8 of the blades It. This second casing ring 28 is of frusto-conical form and at its rear edge presents a radially outwardly extending flange 26 interengagement with the rear edges of the blade platforms I! already in place, through a stepped formation 21a. The shroud formed by the ring of sections 21 is spaced from the outer casing ring to define therewith an air cavity 30, having an inlet port 3| over the stepped formation 28, and has at its rear edge a short outward radial flange 32 with ports 33 therein. The radial flange 32 also has a peripheral forwardly directed flange engaging a corresponding peripheral groove in the second casing ring 20, thereby, with the stepped formation 28, supporting the shroud 21 in radially fixed relationship to the ring 20.

The third low temperature casing ring 35, in the main, is again of frusto-conical form and has set back slightly from its forward edge a radially outwardly extending flange 36 at whose periphery is a forwardly extending annular flange 31 which, with the flanges 36, 26 defines an annular chamber of generally rectangular channelled section in which are accommodated the roots 3% of a row of stator blades 39. The roots 38 are located in the channel formation by bolts 40 engaging axial grooves therein, which bolts also serve to secure together the flanges 36, 26. The roots 38 are further positioned by the interengagement therewith of an axially raised rib on the flange 36. This rib also, with the edge of the ring 35, defines an air cavity 4| for cooling purposes. The roots 38 are themselves recessed to form coolant passages, as indicated at 42 and in dotted lines.

The rear edge of the third casing ring 35 has a radial flange 43 surrounding an undercut groove, similarly to the rear edge of the second ring 20, the said groove receiving a forwardly directed flange 44 at the rear edge of a second high temperature inner shroud ring 45 which is similar in its construction to the first shroud ring 21 and is supported by the interengagement of its forward edge with the blade root 38. As in the case of the first shroud ring, a coolant space 45 is left between the second shroud ring 45 and the third casing ring 35, the space having an inlet port 41 by which air is received from the passages of the blade root 38, and an outlet port 48.

The rear face of the second shroud ring 45 has attached to it an extension piece 49, which is of similar construction to the shroud ring 45 in that it is in peripherally discontinuous sections interconnected by expansion joints. Said extension piece 49 forms a liner for the exhaust duct wall 50, which has a flange and bolt attachment with the flange 43 of the third casing ring.

The nozzle ring stator blades l6, as well as being supported at their outer (root) ends by the first casing ring 13, are located at their inner (tip) ends by a composite inner nozzle ring built up from two angle rings 5!, 52, bolted to a member of the inner wall 3 of the aircasing 2, and having flanges 53, 54, defining seatings which are engaged with a radial clearance to allow for expansion by a spigot member formed at the tip of each blade, said spigots acting both to position the blades and react the gas loads'thereon. The tips of the blades 16 are also formed with platforms 56 which abut peripherally to form a virtually continuous floor for the nozzle passage and also enclose beneath them a chamber 51 for cooling air, which may enter by way of an inlet at 58 between the aircasing structure and platforms 56 from the main air stream emerging from the air-casing, and be discharged at 59 to the slight gap between the rotor and the rear edge of the nozzle ring 5|.

It will be appreciated that the inner high temperature shrouds 21, 45, by virtue of their peripherally effective expansion joints are capable of maintaining a constant diameter notwithstanding temperature variations; further, by virtue of their attachment to the low temperature outer casing rings 13, 20, 35, their actual diameter at any time is determined by the diameter of said casing rings; and finally that by reason of the circulation of cooling air from the chamber 23 through the cavities between the shroud rings and outer casing rings the temperature variation of the latter and the consequential variation in their diameter, and therefore, also, in that of the shroud rings can be kept within quite narrow limits, thus enabling a smaller tip clearance to be left between the rotor blades and shrouding when cold.

This clearance can be still further reduced by cooling the rotor of the turbine 6, which for this purpose comprises axially spaced forward and rear rotor elements 60, 6|, of which the former has a hollow hub 62 by which it is directly attached to the shaft 5, which is also made hollow, whilst the latter is mounted on a rearward extension 63 of said hub 62 and has in its own hub axial passages 64 which communicate with the interior of the hollow hub extension 63 and shaft 5 and also with a gap left between the two rotors 60, 6 I, and with radial passages between the rear face of the rotor 6| and a nut 65 by which said rotor is held on said hub extension 63. The spaced rotor elements 60,, 6|, are reinforced by an annular disc 66 having at its inner edge an enlarged ring formation 6'! which engages the inner faces of annular ribs 68 on the rotors 60, 6|, these ribs being ported at 69. At its outer edge the disc 66 has a flanged rim 10 which occupies the axial gap between blade mountings in the rims of the rotors 60, 6|, to form a floor for the working fluid passage, whilst inwardly of its rim 10, the disc 66 has annular ribs H having driving dogs thereon engaging complementary dogs on ribs 12 on the rotors, allowance being made for the passage of air through the dogs to the chambers 13 thus formed flanking the blade roots of the two rotors, and thence through or under the blade roots. In order to avoid internal stressing of the disc 66 due to unequal expansion of its rim 10 which is heated due to its exposure to the hot working gases, and its main body, which remains relatively cool, the disc 66 is peripherally interrupted by short radial slots (not seen in the drawings) which, for the purpose of preventing leakage of cooling air direct from the chambers 13 to the working gas stream, instead of through or under the blade roots, are constructed so that under working conditions they are closed at their radially outer ends. This may be achieved by making the slots as narrow saw cuts whose edges are burred over to close the slots by a hammering operation prior to the final machining of the rim 10. The weak flanges thus formed are crushed at the first expansion of the rim so that the slots, though open when the disc 66 is cold, are closed at the working temperature. It is desirable, in order to maintain a balanced flow of air past the blade roots in the rotors 50, 6|, to prevent leakage from one of the chambers 13 to the other. This is achieved by the provision of suitable sealing means such as grubscrews in the radially inner parts of the slots and by plugging the drilled holes which, in accordance with normal practice, would form the radially inner ends of the slots. Similar provision for differential expansion of the rotors ill, 6!, is not required due to the existence of the blade root mountings.

The forward face of the rotor 60 is screened by an air guide 14 which is mounted on a fixed part of the bearing 15 of the shaft and defines a narrow chamber extending along the greater part of the radius of the rotor 50. A similar air guide 16 is formed by a part of the exhaust structure adjacent to the rear face of the rotor 6|.

It will be remembered that the chamber I enclosed by the inner wall 3 of the aircasing is maintained under pressure. The stationary parts of the shaft bearing have air seals at 11 with the shaft 5 which permit the leakage of air into the bearing housing and thence not only through the bearing itself, but also through a passage I8 to a chamber 19 surrounding the hub of the rotor 60. There is thus a constant flow of compressed air outwardly between the face of the rotor 60 and the air guide 14, the pressure being maintained higher than that of the working fluid in the turbine so that the air will pass into the turbine between the nozzle ring and the first rotor 60. In addition, the shaft 5 is ported to receive compressed air from the chamber 1, and this passes by way of outlet ports 80 to the passages 64 in the hub of the rotor GI and thence up the inner faces of the two rotors and the rear face of the rotor, eventually leaking into the working fluid as before. By these means expansion and contraction of the turbine rotor as a whole can be materially reduced, thus enabling the blade tip clearances in turn to be reduced as compared with the case of an uncooled rotor and, in conjunction with the compensated stator construction already described resulting in an extremely high efficiency from the point of view of tip leakage losses.

I claim:

1. A turbine, compressor. or like machine, especially an internal combustion turbine operating at high temperatures, having radially inner and outer structures defining inner and outer boundary walls of an annular axial flow working fluid channel, and an element in said channel which has to have a radial clearance from at least one wall thereof, both the walls and the element being required to be subject to thermal radial expansion and contraction; at least one of said boundary walls comprising a shroud ring exposed to the fluid in and forming a high temperature boundary surface of said channel, and a lower temperature shroud ring-supporting structure screened from the channel by said shroud ring, said shroud ring comprising a plurality of peripherally discontinuous sections having slots at their axially extending edges and mounted on said supporting structure in fixed radial relation thereto so that the radius of the ring is determined thereby, and said sections having between the expansion joints which close the spaces between successive sections to provide for its peripheral continuity and are effectlve;to accommodate expansion and contraction of said sections in the peripheral direction, each of said expansion joints being formed by a strip element which engages in slot in axial edges of a pair of adjacent peripherally discontinuous sections of said shroud ring.

2. A machine as claimed in claim 1, wherein the high temperature shroud ring and the lower temperature outer structure have mutually interengaging formations by which the shroud ring is rigidly locked in the radial sense to the outer structure, said interengaging formations comprising a peripherally extending axial directed flange on the one part and a corresponding peripheral recess on the other, said flange mating with said recess to define the radial location of the shroud ring.

3. An axial flow turbine, compressor, or like machine, especially an internal combustion turbine operating at high temperatures, having a bladed axial flow rotor defining an inner boundary wall of an annular axial flow working fluid channel; an outer annular structure enclosing said blading and defining an outer boundary wall of said channel; said outer structure comprising a radially inner shroud ring exposed to the fluid in and forming the outer high temperature boundary surface of said channel, and a lower temperature shroud ring-supporting structure screened from the channel by said shroud ring, said shroud ring comprising a plurality of peripherally discontinuous sections having slots at their axially extending edges and mounted on said supporting structure in fixed radial relation thereto so that the radius of the ring i determined thereby, and said sections having between them expansion joints which close the spaces between successive sections to provide for it peripheral continuity and are effective to accommodate expansion and contraction of said sections in the peripheral direction, each of said expansion Joints being formed by a strip element which engages in slots in axial edges of a pair of adjacent peripherally discontinuous sections of said shroud ring.

4. An axial flow turbine, compressor, or like machine, especially an internal combustion turbine operating at high temperatures, having a bladed axial flow rotor defining an inner boundary wall of an annular axial flow working fluid channel; an outer annular structure enclosing said blading and defining an outer boundary wall of said channel; said outer structure comprising a radially inner shroud ring exposed to the fluid in and forming the outer high temperature boundary surface of said channel, and a lower temperature shroud ring-supporting structure screened from the channel by said shroud rin said shroud ring being mounted on said supporting structure in fixed radial relation thereto so that its radius is determined thereby, and expansion gaps being embodied in the shroud ring which accommodate its expansion and contraction in the peripheral direction; said lower temperature shroud-supporting structure being built up of a plurality of ring elements inter-attached in axial succession to form a rigid unit and having blading mounted thereon to extend radially inwardly into the working fluid channel through said inner high temperature shroud ring, said blading being mounted in root anchorages presented by formations of adjacent axially inter-v attached ring elements of said outer structure upon their juxtaposition, and said anchorages fluid channel downstream of said rotor blading,

said further structure being separably attached to the downstream end of said extension; and a shroud ring surrounding but not substantially exceeding in axial extent said rotor blading, said shroud rin extending from the downstream side of said nozzle blading to the downstream end of said extension and comprising a plurality of separate arcuate elements free to expand peripherally and being supported in fixed concentric relation to said stationary structure and extension by annular seating means at its upstream and downstream edges effective to afford uniform peripheral distribution of such support while allowing freedom of expansion to said arcuate elements in the peripheral direction; said shroud ring being radially spaced from said extension and defining therewith an enclosed chamber.

6. An axial flow turbine, compressor, or like machine, especially an internal combustion turbine operating at high temperatures, having at least one annulus of stator blading and at least one annulus of rotor blading downstream thereof mounted on a radially inner rotor; stationary structure axially substantially coextensive with and annularly surrounding said one annulus of stator blading and said one annulus of rotor blading, said stator blading being supported by attachment to said stationary structure, further stationary structure surrounding the downstream continuation of the working fluid channel through said blading and attached to said first mentioned structure; a shroud ring spanning axially the region from the outlet of said stator blading to the downstream edge of said first mentioned structure, said shroud ring forming the immediate boundary of the working fluid channel in that region and surrounding said rotor blading; said shroud ring comprising a plurality of separate arcuate elements between which are expansion gaps and each having slots at their axially extending edges and being supported'in fixed concentric radial relation to said first mentioned structure by annular seating means at the upstream and downstream edges of the ring which are effective to afford uniform peripheral distribution of such support while allowing freedom of expansion to the elements of the ring in the peripheral direction; and expansion joints which close the spaces between successive elements of said shroud ring and are effective to accommodate expansion and contraction of said elements in the peripheral direction, said expansion joints being formed by strip elements engaging in slots in the axial edges of pairs of peripherally adjacent elements of the shroud ring.

7. An axial flow turbine, compressor, or like machine, especially an internal combustion turbine operating at high temperatures, having at least one annulus of stator blading and at least one annulus of rotor blading downstream thereof mounted on a radially inner rotor; stationary structure axially substantiall coextensive with and annularly surrounding said one annulus of stator blading and said one annulus of rotor blading, said stator blading being supported by attachment to said stationary structure; further stationary structure surrounding the downstream continuation of the working fluid channel through said blading and attached to said first mentioned structure; a shroud ring spanning axially the region from the outlet of said stator blading to the downstream edge of said first mentioned structure, said shroud ring forming the immediate boundary of the working fluid channel in that region and surrounding said rotor blading; and said shroud ring comprising a plurality of separate arcuate elements each being supported in fixed concentric spaced radial relation to said first mentioned structure by annular seating means at the upstream and downstream edges of the ring which are effective to afford uniform peripheral distribution of such support while allowing freedom of expansion to the elements of the ring in the peripheral direction.

8. A turbine according to claim 7 wherein said shroud ring and said first mentioned stationary structure define therebetween a cavity, and means are provided for circulating a coolant through the cavity to directly cool said shroud ring.

9. A multistage axial flow turbine. compressor. or like machine, especially an internal combustion turbine operating at high temperatures, comprising at least two stages of rotor blading mounted on a radially inner rotor to define the inner boundary of a working fluid passage through the blading; stator blading extending radially inwardly between said stages of rotor blading to constitute an intermediate stator stage in relation thereto, stationary structure annularly surrounding said rotor blading and comprising in respect of two successive stages, each consisting of one stage of stator and rotor blading, successive ring units each substantially coextensive in the axial direction with one set of stator and rotor blading. successively adjacent ring units presenting therebetween root anchorages for mounting atleast one stage of said stator blading, and shroud rings positioned respectively upstream and downstream of said stator blading and forming in conjunction therewith an axially continuous outer boundary of the working fluid passage, each shroud ring comprising a plurality of separate arcuate elements between which are expansion means and being supported in fixed concentric radial relation to said stationary structure by annular seating means at the upstream and downstream edges of the ring effective to afford uniform peripheral distribution of such support while allowing freedom of expansion to the elements of the ring in the peripheral direction.

10. An axial-flow gaseous-fluid turbine, especially a gas-turbine operating at high temperatures, comprising a rotor incorporating at least one ring of rotor blades, a stator structure enclosing the rotor, incorporating at least one ring of stator blades upstream of the rotor blades, and, within said stator structure, a stationary shroudring constituted by a ring of separate, short, axially straight arcuate elements enveloping said ring of rotor blades as an outer boundary of the gas path through the turbine blading and spanning only the one ring of rotor blades, the arcuate elements being separated by expansionpermitting means, each edge of each arcuate ele ment being held on the stator structure by a ture into which fits an axially-extending part of the other of said arcuate element and stator struca,se4,soo

ture, so that thermal expansion of the shroud 5 ring can take place circumierentially but not in diameter, and said shroud ring and stator structure denning therebetween a cavity separate from the gas path through the turbine blading for receiving a coolant to directly cool said shroud ring.

11. A turbine according to claim 9 wherein the expansion permitting means between two adjacent arcuate elements is constituted by the axially extending edge of one element overlapping the corresponding edge of the other element.

12. A turbine according to claim 9 wherein the expansion permitting means between two adjacent arcuate elements is constituted by the axially extending edge or one element entering a groove in the corresponding edge of its neighbor.

RODERICK CRIB'I'ALL Mal-DOD.

12 mmnanncns crrnn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,325,135 Baumann Dec. 16, 1919 1,690 737 Hodgkinson Aug. 14, 1928 1,873,743 Doran Aug. 23, 1932 1,960,810 Gordon May 29, 1934 2,241,782 Jendrassik May 13, 1941 2,445,661 Constant et a1. July 20, 1948 2,472,062 Boestad et a1. June 7, 1949 FOREIGN PATENTS Number Country Date 243,974 Great Britain Dec. 10, 1925 542,197 Great Britain Dec. 20, 1941 29,236 France Mar. 10, 1925 (Addition to No. 583,702) 

